Superchargers have long been known and used to increase the power output of internal combustion engines without increasing the engine displacement. Superchargers, as referred to herein, generally include both mechanically driven and exhaust driven mechanisms. The amount of power developed by an engine is often limited only by the air mass that can be inducted into the engine cylinders to stoichiometrically react with the provided fuel. Compression of the incoming air charge by supercharging increases the density of the air charge, thus providing additional air mass per unit volume within the engine combustion chamber. This additional air mass allows the introduction of proportionally greater fuel mass as stored chemical energy, hence increasing the net engine power output without increasing the size of the engine or modifying the engine displacement.
The supercharging process also tends to significantly increase the temperature of the compressed air charge above ambient temperatures (air temperatures exceeding 300.degree. F. are not uncommon). This increase in air temperature tends to lower the density of the air charge, with adverse consequences to the efficiency of the supercharger. Heat exchangers (referred to hereinafter as aftercoolers) are therefore often used in conjunction with superchargers to reduce the temperature of the compressed air charge to further increase the air change density.
An aftercooler is also effective against the deleterious effects of high air charge temperature on the operation of the engine. These high initial temperatures contribute to extreme final combustion temperatures, thus contributing to excessive thermal loading and high concentrations of NOx emissions created at very high air temperatures. Further, at least in spark-ignition engines, the elevated temperatures within the cylinder encourage auto-ignition, thereby contributing to excessive hydrocarbon emissions and lower engine efficiency. The supercharger and aftercooler, combined in the induction system, provide a cool, dense air charge to the engine to avoid these problems.
The benefits of supercharging and aftercooling are not uniformly realized at all operating conditions, however. At very low ambient temperatures, the air discharged from the supercharger is heated only moderately. Typical air-to-air aftercoolers, to which this invention is addressed, tend to reduce this air charge temperature to an even lower, and frequently undesirable, temperature.
Indeed, the air charge temperature obtained in conventional supercharger/aftercooler systems can be low enough to chill the cylinder wall surfaces as well as the air/fuel mixture in recently started engines. This cooling of the mixture, especially near the cylinder wall, prevents full combustion of the mixture. Thus, the hydrocarbon emissions of a "cold" engine increase, while the engine efficiency decreases. Also, the low air charge temperature entering the engine tends to lower the heat energy flowing to the engine coolant, which in turn tends to reduce the amount of heat energy available for other uses, such as heating the passenger compartment.
Further, as the ambient air temperature decreases, the air charge is initially denser. Consequently, the air charge pressure after supercharging and normal aftercooling is quite high, as is the engine cylinder pressure throughout the engine cycle. The increased pressure often exceeds the design strength specifications for the engine components and causes mechanical failures.
The invention thus is aimed at altering the operation of the aftercooler at cold ambient temperatures by reducing the efficiency of the aftercooler and negating some of the work of the turbocharger by imposing a pressure drop across the aftercooler.